1. Field of the Invention
The present invention relates to the field of semiconductor fabrication technology and, in particular, to the formation of substrates having multiple layers, i.e., the formation of multilayer substrates.
2. Description of Related Art
Multilayer substrates are becoming increasingly popular in the microelectronics arena for a variety of reasons. The high packing densities, accommodation of varied types of circuit elements and facilitation of complex circuitries afforded by multilayer substrates have generated an increased interest and use in these devices. As the requirements for increased packing densities, complex circuit functionality and other requirements necessitated by current market conditions proliferate, so too will the demand for multilayer substrates that enable such requirements.
A number of techniques currently exist for multilayer substrate fabrication. One method of fabricating multilayer substrates is to laminate single, unfired semiconductor tapes, commonly referred to in the industry as “green sheets” or “green tape.” In a typical, conventional layered green sheet process, a multilayer substrate may be produced by a) printing a circuit pattern on a glass or ceramic green sheet; b) laminating several green sheets that have been printed; and c) firing or sintering the laminated green sheets in a reducing atmosphere. The resulting product is a laminated, multilayer substrate that may be used in a variety of applications.
Multilayer substrates formed from green sheets have a number of disadvantages. Although any number of green sheet layers may be laminated together, the overall height of the multilayer substrate is determined by the thickness of each individual green sheet used in the multilayer structure. Green sheet thickness is determined by the green sheet manufacturer, and, typically, the thickness of an individual green sheet is in the area of 6 mils to 30 mils. Most conventional multilayer substrate fabrication techniques cannot accommodate green sheets of less than 6 mil thickness because green sheets of these thicknesses tend to be too thin to be successfully fabricated into a multilayer substrate. Thus, where a multilayer substrate having thin layers is desired, green sheets are generally not feasible as an option.
Also, because the end user has no control over the thickness of each individual green sheet, the end user is forced to limit the number of layers of the multilayer substrate according to the maximum height allowable for the intended application of the multilayer substrate device. For applications requiring minimum substrate height, the number of green sheets available for lamination may be limited due to their predetermined thickness, thus limiting the complexity of the design. Accordingly, for small footprint devices that require complex designs, laminated green sheets may prove inadequate.
Another disadvantage of using laminated green sheets for multilayer substrate fabrication is that the green sheets must be sintered, or fired during the fabrication process. As is the case with many physical processes requiring the introduction of heat, firing the laminated green sheet, i.e., heating the green sheet, causes shrinkage in the various elements of the substrate, including the circuit traces printed on each laminate. Thus, the tolerances, dimensions and other parameters of the traces and other elements of the substrate are subject to variance. Moreover, the exact amount of shrinkage during firing may be difficult to control. For designs in which geometries are critical, requiring minimum variance in trace tolerances and high accuracy in trace dimensions or other substrate parameters, laminated green sheets may prove wholly inadequate. If a multilayer substrate is fabricated using green sheets in which each of the individual green sheets have thicknesses of varying heights, a technique which is common in the semiconductor industry, the effects of firing and shrinkage are even more pronounced and dramatic.
Another disadvantage of using laminated green sheets for multilayer substrate fabrication is that the dielectric used to form the green sheets is generally not of as high a quality as is required for some applications. Because multilayer substrates fabricated with green sheets are subjected to elevated temperatures during processing, the dielectrics used in green sheets must be able to tolerate elevated temperatures without compromise to the integrity of the dielectric. Accordingly, impure dielectrics, which typically show enhanced reliability at elevated temperatures, are used as the dielectric in green sheets. These impure dielectrics can be limiting when forming circuit elements, such as high voltage capacitors for example. Thus, in applications where pure dielectrics are required, the temperature processing required for multilayer substrates fabricated using green sheets generally precludes green sheets as an option for forming such substrates.
Another method of fabricating multilayer substrates is to use thick film processes. The thick film process is a sequential process requiring a) printing a circuit pattern on a substrate; b) drying the substrate; and c) firing the substrate. This process is repeated as many times as is necessary or desired in order to obtain the number of layers required. Each individual substrate is printed, dried and fired, then adhered to the other substrates which have been printed, dried and fired to form a substrate stack. In this way, a multilayer substrate is formed from individually prepared substrates.
However, multilayer substrates formed from thick film processes suffer from many of the same disadvantages as green sheet multilayer substrates. The thickness of a multilayer substrate formed using thick film processes is dictated by the thickness of each individual substrate. As is the case with green sheets, the ultimate height of the substrate is determined by the cumulative height of the substrates rather than the requirements of the application. Excessive cumulative substrate height may be an unacceptable design restriction and, thus, thick film processes may be inadequate for forming multilayer substrates in the intended application.
Because thick film processes require firing each substrate layer, multilayer substrates formed using thick film processes also suffer from shrinkage issues. Like green sheets, designs requiring minimum variance in substrate parameter tolerances and high accuracy in substrate parameter dimensions may not be adequately served by thick film multilayer substrate formation processes.
The filling of vias in the substrate is a normal part of the substrate fabrication process. Traditional via filling is usually accomplished using thick film printing techniques, where squeegee printing processes push conductive ink into the vias, or with vacuum techniques that effectively pull conductive ink through the vias.
When conductive ink is pulled through a via, much of the ink is lost due to waste. For small production runs, such waste may be of little concern. However, for large production runs, the cost attributed to wasted ink could ultimately exceed the cost of the substrate itself. Thus, for large production runs, filling substrate vias using vacuum techniques becomes cost prohibitive.
Although there is less waste in thick film techniques, traditional squeegee via filling techniques typically allow air to be trapped in the conductive ink, causing air voids to form within the interior of the via. Air voids are deleterious to the conductive path through the via, plaguing the via with a variety of performance detractors. Air voids can greatly reduce the electrical conductance through the path, increasing via resistance and generating excess heat in the via. To complicate matters, air voids can greatly reduce the thermal conductance through the path, providing even less of a mechanism by which to remove the excess heat resulting from the decrease in electrical conductance.
Of equal if not greater concern for practitioners involved with devices that are implanted into a human or animal body, air voids compromise the hermeticity of the conductive path through the via. Often, the conductive path through the via is the single path bodily fluids may take to reach sensitive microelectronics that could be damaged or destroyed by exposure to such fluids. If an implantable device is formed having vias that require hermetic sealing, any air voids within the via that compromise the hermeticity of the path and allow fluids to pass through the via could result in device malfunction or even device failure, which could have disastrous consequences for the user of the device.
Accordingly, there is a need in the semiconductor industry for multilayer substrates having layers which may vary widely in thickness and which may be accurately controlled. There is also a need in the semiconductor industry for multilayer substrates in which circuit element and trace tolerance is tightly controlled and line dimension shrinkage is minimized. There is yet another need in the semiconductor industry for multilayer substrates in which high purity dielectrics may be used.
In addition, there is a need in the semiconductor industry for methods and systems for filling vias in a substrate that do not waste conductive ink. There is yet another need in the semiconductor industry for methods and systems for filling vias in a substrate that eliminate air voids in the conductive path through the via.